Recently, electronic devices have been required to be miniaturized, to be enhanced in efficiency, and to be diversified; and further, when being used for automobile, they have been required to withstand harsh environments exposed at high temperatures. Along with the above demands for the electronic devices, permanent magnets have been increasingly required to possess high performances. To meet these demands, rare earth magnets have been actively developed as permanent magnets, and particularly, in recent years, R--Fe--B based alloys have been focussed as permanent magnet materials exhibiting excellent magnetic characteristics. Thus R--Fe--B based alloy magnetic powders as the sources of the above R--Fe--B based alloys have been developed.
In particular, as a method of fabricating magnetic powders excellent in magnetic properties, there has been remarked a method of fabricating R--Fe--B based alloy magnetic powders by a method wherein R--Fe--B based alloys are subjected to hydrogen absorbing treatment, followed by dehydrogenation. For example, Japanese Patent Laid-open Gazette No. HEI 1-132106 discloses an R--Fe--B based alloy magnetic powder of this type.
The above R--Fe--B based alloy magnetic powder is fabricated by holding an ingot of an R--Fe--B based alloy mainly containing a ferro-magnetic R.sub.2 Fe.sub.14 B type intermetallic compound (hereinafter, referred to as "R.sub.2 Fe.sub.14 B type phase") or a powder of the ingot in a hydrogen atmosphere heated at a high temperature for hydrogen absorption; dehydrogenating at the same high temperature; and dehydrogenating the ingot or the powder thereof in a vacuum atmosphere, to thus generate the R.sub.2 Fe.sub.14 B type phase as a ferro-magnetic phase again. The R--Fe--B based alloy magnetic powder thus obtained has an aggregate structure mainly containing an extremely fine recrystallized structure of the R.sub.2 Fe.sub.14 B type phase having an average grain size of 0.05 to 3 .mu.m, and has high magnetic properties.
However, the R--Fe--B based alloy magnetic powder thus fabricated by the above method, which has the excellent magnetic properties, has a disadvantage that the magnetic anisotropy is significantly reduced and is fluctuated depending on the alloy composition and crystal structure of the ingot and grain size, and on the slight fluctuation of the conditions of the treatments such as homogenization, hydrogen absorption and dehydrogenation. The reduction and deviation of the magnetic anisotropy are extremely inconvenient in the industrial mass-production, and in the worst case, they also make difficult the industrial fabrication.
To cope with this problem, for example, Japanese Patent Laid-open Gazettes Nos. HEI 3-146608 and 4-17604 disclose a technique of heating and hydrogenating an ingot or the like together with a heat reservoir having a heat keeping function, on the basis of a supposition that the deviation of the magnetic anisotropy is generated by the fluctuation in temperature due to the exothermic reaction of the hydrogen absorbing treatment. However, this technique has problems, as being pointed out by Japanese Patent Laid-open Gazette No. HEI 5-163510, such that all the surfaces of an ingot are difficult to be contacted with a heat reservoir; that a furnace must be enlarged to contain the heat reservoir; and that the sticking and entrapment of the fragments of the heat reservoir to the ingot lowers the magnetic characteristics.
On the other hand, the temperature characteristic of R--Fe--B based alloy magnets is poor; for example, the Curie point (Tc) is about 300.degree. C. (370.degree. C. at maximum). Japanese Patent Publication Gazette No. HEI 3-19296 discloses the improvement of the temperature characteristic of R--Fe--B based alloy magnets.
An alloy containing Co as an element for improving the temperature characteristic is pulverized to a powder of 3 to 10 .mu.m. The powder is then compressed and sintered. In the sintered permanent magnet thus obtained, the Curie point which exhibits the improvement in the temperature characteristic is increased; however the residual magnetic flux density is reduced.
In the R--Fe--B based alloy, however, when the Co content is increased, the coercive force (iHc) tends to be reduced, and therefore, the improvement thereof is required.
Moreover, in the applicable range of the recent permanent magnets, resin bonded magnets have been come to be increasingly used, as compared with the sintered magnets. The reason for this is as follows: namely, a resin bonded magnet is fabricated by bonding a magnetic powder with an organic resin or metal based resin and thereby it is inferior in the magnetic properties to a sintered magnet of the same type; however, it is excellent in the mechanical properties to be made easy in its handling, and further, it has the high freedom of the shape. Thus the applicable range of the resin bonded magnets are increasingly expanded along with the development of the magnetic powders of this type having excellent magnetic properties.
The resin bonded magnet is formed by compression molding, extrusion molding, and injection molding. The compression molding is difficult in the integral formation with a result of the reduced freedom of the shape; however, it can increase the space factor of a magnetic powder up to 80 to 90 vol %, to thereby obtain high magnetic properties. The extrusion molding is slightly low in the space factor of a magnetic powder, for example 70 to 75 vol %; however it enhances the magnetic properties, and enables the continuous fabrication. On the other hand, the injection molding enables the integral molding, and excellent in the dimensional accuracy and the freedom of the shape; however, it is limited in the amount of a magnetic powder, for example, 60 to 65 vol % for enhancing the productivity. Accordingly, the injection molding makes it difficult to increase the magnetic performance, which has a limitation to the practical use.
However, as a resin bonded magnet using a rare earth magnetic powder excellent in magnetic properties, an Sm--Co based anisotropic magnet fabricated by injection molding is disclosed in Japanese Patent Laid-open Gazette No. HEI 2-153507, which uses a molding method in which a magnetic powder is pre-magnetized in a magnetic field higher than the molding magnetic field, whereby improving the magnetic properties.
Japanese Patent Laid-open Gazette No. HEI 3-129702 discloses an Nd--Fe--B based magnet excellent in magnetic anisotropy and corrosion resistance, which is fabricated by compression molding.
On the other hand, many techniques on the Nd--Fe--B based magnet have been disclosed on the basis of the recent research for enhancing the magnetic properties of rare earth magnets and for the reason of a problem of resources.
However, with respect to an Nd--Fe--B--Co based resin bonded magnet excellent in magnetic anisotropy and temperature characteristic using an Nd--Fe--B--Co based magnetic powder having an excellent productivity and stable quality, there have not been disclosed any technique of fabricating the above resin bonded magnet by injection molding or compression molding.
A primary object of the present invention is to provide an R--Fe--B based alloy magnetic powder excellent in magnetic anisotropy and a method of stably fabricating the above magnetic powder by suppressing variation in magnetic properties. A further object of the present invention is to provide an R--Fe--B--Co based alloy magnetic powder excellent in magnetic anisotropy and temperature characteristic and a method of stably fabricating the above magnetic powder by suppressing deviation in magnetic properties.
A still further object of the present invention is to provide an R--Fe--B--Co based resin bonded magnet excellent in magnetic anisotropy and temperature characteristic.